Toughened, crack resistant fiber reinforced composite article and method for making

ABSTRACT

A composite article, for example a blading member of a gas turbine engine, comprising a plurality of stacked layers of reinforcing fibers bonded together with a matrix resin is provided with enhanced resistance to impact cracking, material loss and/or delamination though use of a matrix resin including properties comprising a tensile strain property of at least 5% and a K 1c  toughness of at least about 850 psi·inch 1/2 . A method for making such a composite article with such resin comprises providing the layers of reinforcing fibers in a substantially dry, unimpregnated condition. The dry layers are stacked as a preform in a mold cavity and impregnated with the resin to wet and impregnate the dry layers of the preform. Then the resin is cured as a matrix about the fibers and the stacked layers.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a fiber reinforced composite articleand its manufacture, and more particularly, to a fiber reinforcedcomposite blading member including an airfoil, for example a turbineengine fan or compressor blade.

[0002] Components for sections of gas turbine engines, for example a fanand/or a compressor, operating at relatively lower temperatures thansections downstream of the combustion section have been made of resinmatrix composites including stacked, laminated plies. Generally suchprimarily non-metallic composite structures, which replaced heavierpredominantly metal structures, include superimposed layers, sometimescalled plies, reinforced with fibers, which include filaments in itsmeaning, in a variety of configurations and lay-up directions, sometimesabout a core and/or with local metal reinforcement or surface shielding.For elevated temperature applications, a variety of materials are usedfor such fibers, including carbon, graphite, glass, boron, etc., as iswell known in the art. Typical examples of such components madeprimarily of non-metallic composites are reported in such U.S. Pat. Nos.3,892,612—Carlson et al. (patented Jul. 1, 1975); 4,022,547—Stanley(patented May 10, 1977); 5,279,892—Baldwin et al. (patented Jan. 18,1994); 5,308,228—Benoit et al. (patented May 3, 1994); and5,375,978—Evans et al. (patented Dec. 27, 1994).

[0003] As has been discussed in detail in such patents as theabove-identified Evans et al. patent, such non-metallic composites in anaircraft gas turbine engine are subject to damage from ingestion intothe engine and impact on components of foreign objects. Such objects canbe airborne or drawn into the engine inlet. These include various typesand sizes of birds as well as inanimate objects such as hailstones,sand, land ice, and runway debris. Impact damage to the airfoil ofblading members, including fan and compressor blades, as well as damageto strut type members in the air stream, has been observed to cause lossof material and/or delamination of the stacked layers. Such a conditionin a rotating blade can cause the engine to become unbalanced resultingin potentially severe, detrimental vibration.

[0004] The above identified and other prior art have reported variousarrangements and structures to avoid such material loss and/ordelamination of layers. Some arrangements, for example U.S. Pat. No.3,834,832—Mallinder et al. (patented Sep. 10, 1974) and theabove-identified Benoit et al. patent, include use of transverse seamsor fastening devices to avoid delamination of laminated compositestructures using ordinary commercial resin systems as the compositematrix.

BRIEF SUMMARY OF THE INVENTION

[0005] The present invention, in one form, provides a composite articlecomprising a plurality of stacked layers of reinforcing fibers, thelayers bonded together with a matrix resin. The article is toughened andhas enhanced resistance to cracking and layer delamination as a resultof the matrix resin including properties comprising a tensile strainproperty of greater than 5% and a K_(1c) toughness of at least about 850psi·inch^(1/2).

[0006] In another form, the present invention provides a method formaking such a composite article comprising providing the plurality oflayers of reinforcing fibers in substantially dry form, not impregnatedwith a partially cured matrix resin. Sometimes such a partially curedmember is referred to as being in the “green state” or as a “prepreg”.According to a form of the present invention, the dry layers are stackedone upon another into a dry preform shape that then is impregnated withthe above defined matrix resin.

DETAILED DESCRIPTION OF THE INVENTION

[0007] Use of the above-defined toughened, high tensile strain resin asthe matrix of a fiber reinforced composite article comprising stackedlayers of reinforcing fibers, preferably aligned, provides an articleresistant to cracking, crack propagation, and layer delamination. Suchresin matrix obviates the need to provide transverse reinforcement forthe purpose of resisting such delamination.

[0008] The method step, in a form of the present invention, of stackingdry rather than prepreg layers into a dry preform for subsequentimpregnation provides the ability to pre-inspect the dry preforms forpotentially detrimental ply assembly conditions such as wrinkles.Inspection early in the manufacturing method and prior to matriximpregnation and curing enables defect correction such as byrepositioning, redraping and/or smoothing of individual dry layerswithout damaging previously placed layers. The known step of stacking ofprepreg type of layers and then curing the stacked layers, as currentlyis common in the art, enables detection of such defects as wrinkles onlyafter resin matrix curing at which point the defect is fixed in thearticle. Thus the method of the present invention, enabling such earlydefect detection and correction capability, improves the yield andreduces the cost of manufacturing a fiber reinforced resin matrixlaminated composite article.

[0009] In addition, dry layers of reinforcing fibers used in a form ofthe method of the present invention does not require use of layerbacking sheets or papers commonly used with individual prepreg layers.Currently, such sheets or papers used to maintain separation ofindividual, tacky prepreg layers prior to assembly must be removed asstacking is conducted.

[0010] The fibers defined herein to be in a dry condition are notimpregnated with a matrix resin. However, in some examples, theindividual fibers include a lightly tacky surface material to enable theindividual fibers to maintain a position or relationship with adjoiningfibers in a layer. However, such lightly tacky condition on the fibersis insufficient to bind individually stacked layers together, therebyenabling the above described defect correction in the dry preform, priorto matrix resin impregnation of the dry stacked layers.

[0011] During one series of evaluations of the present invention, heatcurable epoxy forms of the above-defined type of toughened resin,included in forms of the present invention, was compared with othercommercially available epoxy resin systems currently used in the art asmatrix resins for laminated composite articles. Such a comparison wasmade with the matrix resin cured in stacked layers of alignedreinforcing fibers. Currently used cured resin systems, one example ofwhich commercially is available as Dow Chemical TACTIX 123 epoxy resinsystem, have a tensile strain property of about 5% or less, for examplein the range of about 2-5%, in combination with a K_(1c) toughness ofless than about 850 psi·inch^(1/2), for example in the range of about400-500 psi-inch^(1/2). These properties both are typical of currentlyused epoxy resin systems. It was recognized that use of such currentresin systems in certain applications resulted in insufficientresistance to impact damage and delamination caused by ingestion of suchobjects as birds, hail, and land ice into the engine. For example, onetype of test impacted the cured laminated structure with an objectequivalent to a 2.5 pound bird. The lower tensile strain of currentsystems was insufficient to resist impact damage or loss of material,and the lower toughness level provided too low a threshold at whichcracking and layer delamination could be initiated. This lower thresholdresults in unacceptable component matrix loss causing performance andbalance conditions detrimental to the engine. In contrast according tothe present invention, a matrix, for stacked reinforcing layers, of acured epoxy resin system with a tensile strain property of greater than5%, for example about 7% or more, in combination with a toughness K_(1c)of at least about 850 psi·inch^(1/2), provided resistance todegradation, such as material damage and delamination, from such animpact.

[0012] In one specific evaluation series, a plurality of shaped layersof dry, substantially unidirectional carbon fiber bundles, commerciallyavailable as IM-7 12K tow tape from Hexel Company, were disposed in astack as layers of a preform in the cavity of a commercial resintransfer mold, with typical amounts of intermediate wicking felt as usedin the art. The mold cavity was in the shape of a turbine engine bladeincluding an airfoil. The preform and its layers were inspected prior toinjection of the resin matrix and any detrimental wrinkles were removedby redraping or smoothing the individual layers while the preform was inthe dry state.

[0013] After closing the cavity of the transfer mold, which includedtypical resin ports and vents, a vacuum was provided in the cavity toremove ambient air from the cavity and from about the preform. Then theabove-defined type of high strain, toughened epoxy resin, identified as3M PR520 epoxy resin system, was injected into the mold cavity about thestacked layers, wetting and saturating the preform with the resin.Properties of that resin system included a tensile strain of about 6.9%and a toughness K_(1c) of about 1380 psi·inch^(1/2). The resin wasinjected into the evacuated mold cavity under a pressure in the range ofabout 25-100 psi.

[0014] After injecting the resin, the mold and its contents were heatedin the range of about 350-400° F. for about 90-120 minutes to cure theresin into a matrix about the fibers into a carbon/epoxy compositeblade. After curing, the mold and its contents were cooled. When removedfrom the mold cavity, the resulting article was a near net shape moldedepoxy resin carbon fiber reinforced composite blade including a moldedairfoil and molded base. The co-cured fiber reinforced composite bladeusing the above-defined high tensile strain, toughened epoxy resin as amatrix provided improved impact capability at the point of impact aswell as away from the impact site while retaining blade spanwise andchordwise directional strength capability.

[0015] In one evaluation to demonstrate the benefit of the presentinvention, two series of geometrically identical flat panels wereprepared in the above-described manner. A first series of panels wasmade with an ordinary epoxy resin system having a K_(1c) tensiletoughness of 800 psi·inch^(1/2). A second series of panels was made,according to a form of the present invention, with the above-describedhigh strain, toughened epoxy resin system having a K_(1c) tensiletoughness of 1380 psi·inch^(1/2). Each series was tested using 2″diameter reinforced hailstones shot at a velocity of 680 feet per secondto impact each panel at an angle to the panel surface. As a result ofsuch testing, the first series of panels, made with the low tensiletoughness resin, exhibited significant through panel material loss andextensive delamination. The second series of panels, made with the highstrain, toughened epoxy resin exhibited no material loss and minimalpoint of impact delamination.

[0016] Embodiments of the present invention, including a method formaking, provide a tough, impact and delamination resistant fiberreinforced resin matrix composite article, for example a blading memberof a gas turbine engine. Resin properties are tailored to provide highimpact resistance while maintaining, for example in an airfoil, spanwiseproperties to resist centrifugal and bending loads as well as chordwiseproperties to resist gas and torsional bending loads. This combinationof enhanced properties improves the overall operating capability of thearticle. The method form of the present invention improves theproduction cycle of the article by substantially eliminating yieldproblems associated with wrinkles which have been seen to occur instacked prepreg layers of composite materials.

[0017] The present invention has been described in connection withspecific examples and combinations of materials and structures. However,it should be understood that they are intended to be typical of ratherthan in any way limiting on the scope of the invention. Those skilled inthe various arts involved, for example technology relating to gasturbine engines, to fiber reinforced composite structures, fibers andresins, etc, will understand that the invention is capable of variationsand modifications without departing from the scope of the appendedclaims.

What is claimed is:
 1. A composite article comprising a plurality ofstacked layers of reinforcing fibers, the layers bonded together with amatrix resin, wherein: the matrix resin includes properties comprising atensile strain property of greater than 5% and a K_(1c) toughness of atleast about 850 psi·inch^(1/2).
 2. The article of claim 1 in which thereinforcing fibers in a stacked layer are substantially aligned with oneanother.
 3. The article of claim 2 in which: each of the stacked layerspredominantly includes substantially aligned reinforcing fibers; and,the matrix resin is an injectable epoxy resin.
 4. The article of claim 3in which the fibers comprise at least one selected from the groupconsisting of carbon, graphite, glass, and boron fibers.
 5. The articleof claim 1 in the form of a turbine engine blading member including anairfoil in which at least the airfoil comprises a plurality of shaped,stacked layers of reinforcing fibers impregnated and bonded togetherwith the matrix resin.
 6. The blading member of claim 5 in which thereinforcing fibers in a stacked layer are substantially aligned with oneanother.
 7. The article of claim 5 in which: the fibers comprise atleast one selected form the group consisting of carbon, graphite, glass,and boron fibers; and, the resin is an injectable epoxy resin.
 8. Amethod for making a composite article comprising a plurality of stackedlayers of reinforcing fibers bonded together with a matrix resincomprising the steps of: providing a plurality of layers ofsubstantially dry, unimpregnated reinforcing fibers, stacking the layersone upon another into a preform shape; and, impregnating the preformshape with a resin that includes properties comprising a tensilecapacity of greater than 5% and a K_(1c) toughness of at least about 850psi·inch^(1/2).
 9. The method of claim 8 in which the reinforcing fibersin a stacked layer substantially are aligned with one another.
 10. Themethod of claim 8 in which: the layers of the preform shape are stackedin a cavity of a mold; the mold cavity is closed; a vacuum is providedwithin the mold cavity about the layers of the preform shape; the matrixresin is provided in the cavity about the fibers and the layers to wetand impregnate the layers and fibers; and, the resin is cured about thefibers and layers.
 11. The method of claim 10 in which: the resin is aninjectable epoxy; after providing the vacuum in the mold cavity, theresin is injected into the mold cavity under a pressure in the range ofabout 25-100 psi; and, the resin is cured at a temperature in the rangeof about 350-400° F.